Antireflective coating films

ABSTRACT

Enhanced fidelity of pattern transfer of aqueous developable photoresist compositions is achieved with top antirefective coatings which are fluorine-containing and have a refractive index approximately equal to the square root of the underlying photoresist and which are removable in the developer for the photoresist.

This application is a continuation of application Ser. No. 08/516,117filed Aug. 17, 1995, now abandoned, which is a continuation ofapplication Ser. No. 08/240,289, filed May 9, 1994 now abandoned, whichis a continuation of application Ser. No. 07/722,773, filed Jun. 28,1991 now abandoned.

FIELD OF THE INVENTION

This invention relates to compositions which enhance the performance ofaqueous developable photoresists by substantially eliminating patterndistortion caused by reflections over non planar features in theworkpiece being patterned. More particularly, the invention is directedto antireflective top coatable compositions which are easy to apply,provide better process control of image formation, are aqueous solubleand thus are removable within the existing lithographic processing, andminimize environmental hazards and do not significantly add to processcosts.

BACKGROUND ART

The use of antireflection coatings for optical devices such as lenses inphotographic and other equipment is well known. These coatings takeadvantage of the relationship described in the Fresnel formulae. It iswell established that the refractive index of an overlying materialshould be at about the square root of the refractive index of theunderlying material and that the thickness of the coating or layershould be an odd integer multiple of one-quarter of the wavelength ofthe incident radiation, the so-called "quarter-wavelength thickness".

In U.S. Pat. No. 4,759,990 to Y-T. Yen, the antireflective coatingconcept is extended for use over optical membrane elements, i.e.,pellicles, which may be used in the manufacture of semiconductor wafers.The pellicle material is typically a nitrocellulose film which isstretched over a pellicle holder (an upstanding ring or the like). Theuse of an aromatic vinyl polymer as a first coating layer such aspolyvinyl naphthalene, polymethylstyrene or polystyrene, followed by afluorocarbon layer such as 3M's FC-721 or FC-77 is disclosed. Pelliclesmay be used a number of times before they suffer either mechanical breakdown or cover contamination. Such pellicles are either cleaned orreworked/replaced.

Tanaka et al., J. Electrochem. Soc., 137, 3900 (1990) (Tanaka I)provides a technique for directly applying an antireflective coating ontop of a resist (ARCOR). This technique seeks to overcome the manyproblems which have been encountered due to reflectivity when increasingthe degree of pattern densification in trying to achieve ULSI havingsub-0.5 um geometries.

Light interference leads to linewidth variations and degraded detectionof through the lens (TTL) alignment marks. Tanaka I discloses theformation of an antireflective film on a resist surface which issufficient to suppress the multiple interference effects due to repeatedreflection of incident light in the resist film. The ARCOR film is clearand has its thickness and refractive index optimized and is used in aprocess which entails the steps of coating and baking a photoresist inthe conventional way, spin-coating a film of the ARCOR material on thephotoresist, imaging the composite structure, removing the ARCOR film,and developing the photoresist. The ARCOR process adds the steps ofspin-coating and removal of the ARCOR film. The following materials werecharacterized:

    ______________________________________                                        ARCOR Material   Refractive Index*                                                                         dLW Factor**                                     ______________________________________                                         (1)       perfluoroalkylpolyether                                                                      1.29      10X                                                 polysiloxane   1.43     2.5X                                         (2)       polyethylvinylether                                                                          1.48     1.7X                                                 polyvinylalcohol                                                                             1.52     1.4X                                        ______________________________________                                         *Refractive index in eline (546 nm), gline (436 nm) and iline (365 nm).       **Reduction in linewidth variation due to reflectivity.                  

Using the Fresnel formulae, Tanaka I determined that the refractiveindex of the ARCOR material should be about equal to the square root ofthe refractive index of the imaging resist used. Tanaka I used a resisthaving a refractive index of 1.64 and the ideal ARCOR refractive indexis 1.28. The Tanaka et al. materials fall into two categories: (1) thosewhich suppress reflection effects and which require organic solventstrip; and (2) those which are aqueous strippable, but which providelittle process benefit. (Refractive index .sup.≧ 1.48.)

Tanaka et al., J. App. Phys., 67, 2617 (1990) (Tanaka II) discloses theuse of perfluoroalkyl polyether or spin on glass anddi-propoxy-bis(acetyl-acetonate)titanium as ARCOR materials for singleand bilayer films to control the interface reflectivities as well as themethods to measure such reflectivities. Tanaka II entails the use ofbaking steps to fix the one and two layer ARCORs disclosed. It is silentas to post exposure and develop methods, if any, to remove the ARCORlayer(s).

Tanaka et al., Chem. Abs., 107:87208y (1987) (Tanaka III) is directed tothe subject matter of JP 62 62,520, viz., a process for coating aphotoresist with an antireflective coating which may compriseperfluoroalkyl-polyether, perfluoroalkylamine, orperfluoroalkyl-polyether-perfluoroalkylamine mixed film. Thereflection-preventive film is removed after pattern wise exposure usinga Freon (a chlorofluorocarbon compound) solvent.

The Tanaka materials having the desired refractive indices are moreexpensive to use. First, there is the required additional process stepto remove the ARCOR material. Second, the removal requires an organicsolvent which is expensive to make/purchase and requires expense tosafely handle and dispose of. Third, the nature of Tanaka's solvents asCFCs requires extreme care to protect against environmental damage. Thewaste management aspects of the Tanaka reflection-preventative materialsweighs heavily against their implementation.

In U.S. Pat. No. 4,701,390 to Grunwald et al., a process for thermallystabilizing a photoresist image layer formed on a substrate is provided,wherein the image layer, prior to being subjected to a post-developmentbake, is coated with a protective material which bonds to thephotoresist, but is readily rinsed from the exposed substrate after postbake and which does not interfere with the desired operation of any ofthe subsequent steps of pattern generation including final removal ofthe photoresist image. The protective (thermally stabilizing) materialmay be a compound or a mixture of two or more compounds selected fromchromotropic acid, perfluorocarbon carboxylic acids, perfluorocarbonsulfonic acids, perfluorocarbon phosphoric acids, and alkali metal,ammonium and amine salts of such acids, ethoxylated perfluorocarbonalcohols, and quaternary ammonium salts ofN-perfluoro-N',N"-dialkylamines.

SUMMARY OF THE INVENTION

In order to overcome the deficiencies of the materials and topantireflective processes noted in the Background Art, the inventionprovides coating compositions which provide a refractive index aboutequivalent to the square root of the refractive index of an underlyingphotoresist and which may be applied in a single quarter wavelengththickness and which are further removable in the developer for theunderlying photoresist.

The preferred top antireflecting coating compositions are binary systemsin an appropriate coating solvent wherein there is a film formingpolymer binder which is soluble or dispersible in water or aqueousalkaline solutions and a low refractive index fluorocarbon which issoluble or dispersible in water or aqueous alkaline solutions. Thesecomponents must be compatible with each other and their proportions inthe composition are adjusted to provide a film having the desiredthickness and index of refraction. The binary systems may be mixtures ofthe functional components or polymers in which components havingdifferent functionalities are reacted together.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

We have discovered that an improved ARCOR method is achievable that addsonly one process step to the imaging process that enables the economicalmanufacturable use of antireflection films in optical lithographyprocess without adding the potential for harmful environmental effectsor increased waste disposal costs.

The ARCOR materials are substantially optically transparent to theexposing radiation about UV light which may be in the i-, g-, or h-lineband or deep UV. This transparency is necessary to facilitate thepatterning of the underlying resist.

Materials For Aqueous Processable, Low Index Coatings

The top anti-reflective (TAR) coatings of the invention arecharacterized in that they have both optimized the refractive index ofthe TAR while at the same time maintaining the aqueous processibility ofthe TAR materials.

To overcome the deficiencies of fluorocarbon polymers which do providenearly ideal refractive indexes (on the order of 1.3-1.4) but do notoffer solubility or strippability in aqueous media, we have tailored anumber of different compositions which overcome these deficiencies.

I. Multi-component systems which generally have two components where thefilm forming composition comprises an admixture of functionalizedfluorocarbons with binders that exhibit aqueous solubility, aqueousdispersibility or have aqueous strippability. Two component systems arecomprised of a film forming polymer binder which is soluble ordispersible in water or aqueous alkaline solutions, and a low refractiveindex fluorocarbon compound which is soluble or dispersible in water oraqueous alkaline solutions.

Specific examples of polymer binders: poly(vinylalcohol), poly(acrylicacid), poly(vinylpyrolidone), poly(vinylsulfonic acid Na+ salt),poly(vinylmethylether), poly(ethyleneglycol), poly(alphatrifluoromethylacrylic acid), poly(vinylmethylether-co-maleic anhydride), poly(ethyleneglycol-co-propylene glycol), poly(methacrylic acid).

Specific examples of fluorocarbon compounds: perfluorooctanoic acid-ammonium salt, perfluorooctanoic acid-tetramethylammonium salt, ammoniumsalts of C-7 and C-10 perfluoroalkyl sulfonic acids (sold under thetrade name Fluorad FC-93 and FC-120 respectively), tetramethyl ammoniumsalts of C-7 and C-10 perfluoroalkyl sulfonic acids, fluorinated alkylquaternary ammonium iodides (Fluorad FC-135), perfluoro adipic acid,quaternary ammonium salts of perfluoroadipic acid. The components ofsuch compositions can exist as simple mixtures, or may be in the form ofsalts between the functionalized fluorocarbon and the polymer binder.Examples include mixtures of fluoroalkyl sulfonates, carboxylates, etc.,with aliphatic polymers having aqueous solubility such as polyvinylalcohols or sulfonates, polyacrylic acid and related copolymers,polyethylene and polypropylene glycol homopolymers and copolymers,polyvinyl methyl ether, and polyethyleneimine. An example of a simplemixture is the combination of polyacrylic acid (a water soluble polymer)with a salt of a perfluoro organic acid (such as ammoniumperfluorooctanoate). An example of the salt formed between thefluorocarbon and the polymer is that formed by the combination of aperfluoroalkyl quaternary ammonium cation with an anionic polymer suchas polyacrylic acid. Additional benefit may be derived by incorporatingfluorine in the binder polymer as well.

II. Systems that are comprised largely of a single component that is theresult of a condensation reaction between a functionalized fluorocarbonand an aliphatic polymer. One component systems make use of low indexpolymers which are not soluble in aqueous media initially but arerendered soluble by base catalysis, where the aqueous base developer isthe source of the catalyst.

Examples are fluoroalkyl carboxylates or sulfonates of hydroxyfunctional polymers such as polyvinyl alcohol Aqueous strippability isfacilitated by the hydrolytic instability of the ester linkages.Carbonates, anhydride and related groups are also suitable linkagesbetween the polymer and fluorocarbon groups.

III. Systems which use a catalyst to cleave the bond between afluorocarbon group and a polymeric binder or to depolymerize afluoropolymer where the reaction products are aqueous strippable orpartly or fully volatile. Other approaches to two component systemsinclude the combination of a polymer binder that is not soluble inaqueous media initially, but is rendered soluble during the process bythe action of a catalyst such as an acid, with low index fluorocarboncompounds. Examples of such polymers are polymethacrylic esters such aspoly(t-butylmethacrylate), poly(nonafluoro-t-butylmethacrylate). Incombination with one of the perfluoroalkyl sulfonic acids listed above,the polymer can be rendered soluble in aqueous solutions by the acidcatalyzed cleavage of the ester group during normal resist bakingprocedures, so that it is easily removed during the resist developmentstep. Other polymers with acid clearable groups such as the t-boc group,can be used in a similar fashion. Examples include materials suchfluoro-t-butyl ester or carbonate polymers and fluoroaldehyde polymerswith acid catalysts.

EXAMPLE 1

One equivalent of tetramethyl ammonium hydroxide pentahydrate (18 g) wasreacted with 0.1 equivalent of perfluorooctanoic acid (41 g) in water,to form tetra methylammonium perfluorooctanoate at a concentration of 5percent by weight of the solution.

EXAMPLE 2

Poly(acrylic acid) of molecular weight 2000, was dissolved in water at aconcentration of 5 percent by weight of the solution.

EXAMPLE 3

10 grams of the solution from Example 1 was added to 5 grams of thesolution from Example 2. The mixture was spin coated on a siliconsubstrate. A highly uniform transparent film of refractive index 1.413wasobtained. The film was easily washed away with water.

EXAMPLE 4

12.5 grams of the solution from Example 1 was added to 5 grams of thesolution from Example 2. The mixture was spin coated on a siliconsubstrate. A highly uniform transparent film of refractive index 1.407wasobtained. The film was easily washed away with water.

EXAMPLE 5

Silicon substrates were coated with a positive workingdiazonaphthoquinone photoresist. The thicknesses obtained ranged from10,000-12,000 angstroms.Samples were over coated with the solution fromExample 3, forming a layer thickness of 650 angstroms. The samples wereexposed to radiation of 3650 angstroms using a GCA step and repeataligner. The amount of exposure energy required to give complete filmremoval upon development (dose to clear) was determined. The dose toclear varied 14% over the range of starting resist thicknesses, due tovariable reflectivity. For samples without the 650 angstrom over coat,the dose to clear varied by 36% over the range of starting resistthicknesses.

EXAMPLE 6

Silicon substrates were prepared as in Example 5. Samples were patternedona GCA step and repeat aligner with an exposure wavelength of 3650angstroms, using a resolution target mask of equal line and spacepatterns. The size of the resulting photoresist images, of the 0.7micron mask patterns, were measured with a scanning electron microscope.The sizeof the images of the 0.7 micron mask pattern varied by 0.06microns, over the range of starting resist thicknesses, due to variablereflectivity. For samples without the 650 angstrom over coat the size ofthe images of the 0.7 micron mask pattern varied by 0.15 microns overthe range of starting resist thicknesses, due to variable reflectivity.Thus, the imagesize variation due to changing reflectivity was reducedby a factor of 2.5Xusing this aqueous processible anti-reflectionmaterial.

EXAMPLE 7

Silicon substrates with varying thicknesses of thermally grown SiO₂ wereprepared and coated with a positive working DQN photoresist. Sampleswere overcoated with the solution from Example 3. The photoresist waspatterned on a GCA step and repeat aligner with an exposure wavelengthof 3650 Angstroms, using a resolution target mask of equal line andspace patterns. The size of the resulting images, of the 0.5 micron maskpatterns were measured with a scanning electron microscope. The size ofthe images of the 0.5 micron mask pattern varied by 0.04 microns overthe range of thermal SiO₂ thicknesses due to variable reflectivity. Forsamples without the 650 Angstrom antireflection overcoat, the size ofthe images of the 0.5 micron mask pattern varied by 0.18 microns overthe range of thermal SiO₂ thicknesses, due to variable reflectivity.Thus, the image size variation due to changing reflectivity, was reducedby a factor 4.5× using this aqueous processible antireflection material.

EXAMPLE 8

20 grams of the solution from Example 1 was added to 5 grams of asolution containing 5% by weight poly(methacrylic acid). The solutionwas spin coated on a silicon substrate. A highly uniform transparentfilm of refractive index 1.401 was obtained. The film was easily washedaway with water.

EXAMPLE 9

A solution was made containing the ammonium salt of a perfluoro 7 carbonsulfonic acid (trade name FC-93 from 3M Co.) and a copolymer of ethyleneand propylene oxide (trade name Pluronic F127 from BASF Corp.) in waterata weight ratio of 65:35, with a total solids content of 5% by weight.The solution was spin coated on a silicon substrate. A transparent filmof refractive index 1.43 was obtained. The film was easily washed awaywith water.

While only the preferred embodiments of the present invention aredescribedabove, many potential modifications which fall within thegeneric concept will occur to those skilled in the art upon a reading ofthe present disclosure. Such modifications which are functionallyequivalent to that as herein set forth are within the teaching of thepresent invention as set forth in the claims which follow.

What is claimed is:
 1. An aqueous-soluble, antireflective coating foruse on an aqueous developable photoresist composition comprising:(a) afilm-forming polymeric binder which is soluble in water or aqueousalkaline solutions, said polymeric binder selected from the groupconsisting of polyvinylalcohol, polyacrylic acid, polyvinylpyrrolidone,polyvinylsulfonic acid, polyvinylmethylether, polyethyleneglycol,polyalphatrifluoromethyl acrylic acid, polyvinyllmethylether-co-maleicanhydride, polyethyleneglycol-co-propyleneglycol, and polymethacrylicacid; and (b) a fluorocarbon compound selected from the group consistingof ammonium salts of perfluoroorganic carboxylic acids having from about6 to about 10 carbon atoms, ammonium salts of perfluoroorganic sulfonicacids having from about 6 to about 10 carbon atoms, and fluoroalkylquaternary ammonium salts in admixture with said film-forming polymericbinder, wherein the antireflective coating has a refractive indexapproximately equal to the square root of the refractive index of theaqueous developable photoresist composition, and wherein theantireflective coating is soluble in water or aqueous solutions, andwherein the antireflective coating is removable by dissolution inphotoresist developer.
 2. The antireflective coating of claim 1 whichfurther comprises a solvent which is immiscible with said aqueousdevelopable photoresist composition.
 3. The antireflective coating ofclaim 1 wherein said coating is comprised of a low refractive indexfluorocarbon compound selected from the group consisting ofperfluorooctanoic acid-tetramethylammonium salt, tetramethyl ammoniumsalts of C-7 and C-10 perfluoroalkyl sulfonic acids, fluorinated alkylquaternary ammonium iodides, perfluoro adipic acid, and quaternaryammonium salts of perfluoroadipic acid.
 4. The antireflective coating ofclaim 1 wherein said coating is substantially optically transparent tothe exposure radiation for the underlying resist composition.
 5. Theantireflective coating of claim 1 wherein the antireflective coating hasa refractive index in the range of about 1.3 to 1.4.